A radar apparatus for ships transmits radio waves from a radar antenna, forms sweep data in a polar coordinate system from a detection signal reflected on an object, transforms the data into a rectangular coordinate system and stores the resultant data into an image memory, and displays the data on a display using a raster scanning method.
Such a conventional radar apparatus will be described with reference to the accompanying drawings.
FIG. 14 is a block diagram illustrating a structure of a major portion of the conventional radar apparatus.
An antenna 1 transmits pulses of radio waves to the outside in predetermined transmission cycles, and receives radio waves reflected from objects in a polar coordinate system and outputs a received signal to a receiver 2, while rotating on a horizontal plane in predetermined rotation cycles. The antenna 1 also outputs to an adder 5 an antenna relative azimuth θa where the ship's head is a reference. The receiver 2 detects and amplifies a received signal from the antenna 1, and outputs a resultant signal to an AD converter 3. The AD converter 3 converts this analog received signal into a digital signal (received data) composed of a plurality of bits. First and second sweep memories 4a and 4b each store the digital received data corresponding to one sweep in real time, and outputs the one-sweep data to an image memory 8 by the time when received data obtained by the next transmission is written again.
The adder 5 adds the antenna relative azimuth θa input from the antenna 1 with a ship's head θc measured with a compass 20 to calculate a north-up display sweep azimuth θ=θa+θc and outputs it to a latch circuit 6. Note that, in the case of north-up display, the addition is performed in the adder 5, however, in the case of head-up display, the addition is not performed, and the antenna relative azimuth θa is directly input from the antenna 1 to the latch circuit 6. The latch circuit 6 latches the input sweep azimuth θ simultaneously with the start of sweep to prevent the sweep azimuth from changing during the time when received data (real sweep data) is read from the first and second sweep memories 4a and 4b and is then written into the image memory 8.
A draw address generator 7 creates addresses which designate pixels in the image memory 8 arranged in a corresponding rectangular coordinate system based on a sweep azimuth θ directing outward from a center of sweep and a read position r in the first and second sweep memories 4a and 4b, where the center of sweep is a start address. Specifically, the draw address generator 7 is composed of hardware which realizes the following expressions.X=Xs+r−sin θY=Ys+r−cos θ
X, Y: addresses designating a pixel in an image memory
Xs, Ys: addresses of the center of sweep
r: distance from the center
θ: sweep azimuth
A draw timing generator 10 generates a control signal required for drawing, and outputs the signal to the sweep memory 4, the latch circuit 6, the draw address generator 7, and the image memory 8.
The image memory 8 has a capacity which stores received data corresponding to one cycle of sweeping, as image data. When a display 9 is subjected to raster scanning, the image data is read from the image memory 8 by a display control section (not shown) and is output to the display 9, in synchronization with raster scanning. In this case, if intensity or displayed color is caused to vary depending on the data value of each piece of pixel data in the image data, the operator can confirm circumstances, such as objects around his/her own ship, and the like (see, for example, Patent Document 1).
Patent Document 1: JP 2000-147088A
In such a conventional radar apparatus, in order to prevent coexistence of image data obtained in a current cycle of sweeping and image data obtained in the previous cycle of sweeping, the image data of the image memory 8 needs to be updated every cycle of sweeping.
The rotational speed of sweep, i.e., the rotational speed of the antenna, is typically 12 rpm to 60 rpm. However, the rotational speed may be changed due to, for example, air resistance caused by wind even during one revolution of the antenna 1. On the other hand, the transmission frequency (transmission repetition frequency) of radio waves is typically in the range of several hundreds of kilohertz to several thousands of kilohertz, and is fixed to a predetermined frequency which is mainly set based on a detection range (detection distance). The frequency is set to be high within a short-distance detection range, and is set to be low within a long-distance detection range.
The density of sweep data with respect to one pixel of the image memory 8 is decreased with a distance from the center. Therefore, when only sweep data obtained by a single time of transmission of radio waves is written into the image memory 8, and the rotational speed of the antenna is fast and the transmission frequency is low, pixel data in the vicinity of the center can be updated, but pixel data far in a distance direction cannot be updated. As a result, all pixel data in the image memory cannot be updated during one revolution of the antenna.
Therefore, as illustrated in a timing chart of FIG. 15, a larger number of sweeps than the number of times of transmission are successively generated to draw the image memory. Specifically, the updating of the image memory is performed by determining a sweep direction based on a current antenna azimuth or a sum azimuth of an antenna azimuth and a compass azimuth, irrespective of updating of the sweep memory due to transmission, and writing the contents of the sweep memory into the image memory. Typically, a time required to draw data corresponding to one sweep is caused to be sufficiently shorter than the transmission cycle. If writing of a certain sweep azimuth is ended, a current sweep azimuth is determined again and the image memory is updated. Such an operation is repeatedly performed.
Specifically, sweep data obtained directly from a received signal (hereinafter referred to as “real sweep data”) and sweep data interpolated between adjacent pieces of real sweep data (hereinafter referred to as “interpolated sweep data”) are used to update each pixel data in the image memory 8. In this case, the interpolated sweep data is obtained using the same data as real sweep data stored in the sweep memory 4 immediately before.
In the case where north-up display having stable azimuths is performed in such a radar apparatus, if the course of the ship is suddenly changed, the change amount of the ship's head θc detected by the compass increases, so that an angle between real sweeps becomes so large that an interpolated sweep cannot be formed. FIG. 15 is a timing chart of a conventional radar apparatus, in which a sweep azimuth θn+1 is followed by θn+3, so that an interpolated sweep having a sweep azimuth θn+2 cannot be generated. For example, such a change in the ship's head θc occurs when a small ship is hit by a wave under foul weather.
The ship's head θc, which is analog data, is typically input to an adder after being converted into digital data. However, when this conversion cycle takes a long time, an interval between consecutive ship's heads θc becomes large, i.e., an interval between real sweeps becomes large.
When the interval between real sweeps required to update the image memory becomes large during one cycle of sweeping as described above, it is not possible to form all interpolated sweeps. FIG. 16 is a sweep configuration diagram of image data. In FIG. 16, a thick-line arrow indicates a real sweep, a thin-line arrow indicates an interpolated sweep, and a hatched portion indicates a region in which no sweep is formed. As illustrated in FIG. 16, a portion of sweep data is not updated in the above-described situation, so that image data obtained in the previous cycle of sweeping remains.
In addition, in the case of the above-described conventional radar apparatus, real sweep data and interpolated sweep data based on the real sweep data have the same data value, and therefore, as illustrated in Table 1, data values change suddenly.
TABLE 1sweep azimuthsweep type(angle)sweep datareal sweepθnBinterpolated sweepθn + 1Breal sweepθn + 3Cinterpolated sweepθn + 4Creal sweepθn + 5Cinterpolated sweepθn + 6D
For example, if “B=6” and “C=0”, sweep data suddenly changes from “6” to “0” at a position where the azimuth changes from θn+1 to θn+3.
In FIGS. 17(a)-(d), FIG. 17(a) is an image data configuration diagram in which only real sweep data is represented by numerical values, and FIG. 17(b) is an image configuration diagram in which intensity is changed based on the numerical values of FIG. 17(a). FIG. 17(c) is an image data configuration diagram in which each pixel data in the image memory 8 of the conventional example is represented by numerical values, and FIG. 17(d) is an image configuration diagram in which intensity is changed based on the numerical values of FIG. 17(c).
For example, as illustrated in an A portion of FIG. 17, when a real sweep θm is “6” and a real sweep θn is “0”, all interpolated sweeps therebetween are “6”. Therefore, pixel data values suddenly change from “6” to “0” between the interpolated sweep and the real sweep θn, so that a video in which an end portion of an image is suddenly cut is displayed.
As another example, in FIGS. 18(a)-(d), FIG. 18(a) is an image data configuration diagram when only real sweep data is represented by numerical values, and FIG. 18(b) is an image configuration diagram in which intensity is changed based on the numerical values of FIG. 18(a). FIG. 18(c) is an image data configuration diagram in which each pixel data in the image memory 8 of the conventional example is represented by numerical values, and FIG. 18(d) is an image configuration diagram in which intensity is changed based on the numerical values of FIG. 18(c). Note that, also in FIGS. 18(a) -(d), a higher density of a pixel indicates a higher intensity of the pixel.
As illustrated in FIG. 18, when noise-like image data which is discontinuously present in the distance direction is present in real sweep data, interpolated sweep data following this also becomes the same image data, so that noise is emphasized in the azimuth direction.
In addition, when noise is present at the same position in the distance direction of consecutive real sweeps, noise is further emphasized in the azimuth direction, so that arc-like images are randomly displayed on the display. Particularly, when a gain is increased, it is more likely that noise is present at the same position in the distance direction of real sweeps adjacent in the azimuth direction, so that random arc-like images are likely to be displayed. In addition, a distance in the azimuth direction between sweeps increases with a distance from the center, and therefore, an arc-like image is likely to be more emphasized at a position more distant from the center, so that an unnatural image is likely to be displayed.
In recent years, there has been a demand for a shorter rotation cycle of an antenna in order to detect a high-speed ship. However, there is an upper limit on the transmission frequency of radio waves because of characteristics of the radio wave transmission section. Therefore, the rotation cycle of the antenna is only reduced, but the transmission frequency of radio waves does not change, so that an angle between real sweeps increases, and therefore, an arc-like image is displayed more noticeably.
An object of the present invention is to provide a radar apparatus which can certainly update image data corresponding to one cycle of sweeping by forming an interpolated sweep between adjacent real sweeps irrespective of an interval between the real sweeps.
Another object of the present invention is to provide a radar apparatus with excellent viewability in which noise is not emphasized and an image of an end of an object does not suddenly change.